I suspect typed throws will eventually want some sort of union capability, but I would hope that it'd have the form of a "type function" — a type expression that evaluates to a type by some simple rule — rather than a general anonymous-sum feature. That is, you ought to be able to throw union(E, IOError), and that type will resolve to IOError if E is Never or IOError but fall back to Error if E is anything else. That fallback rule avoids the general pitfalls of these anonymous union types but also make it very special-case around errors.
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